The present invention relates to equalizers having programmable frequency response. In particular, it relates to a programmable equalizer that avoids switched resistor/capacitor (RC) networks that previously were prevalent in the art by providing a transadmittance amplifier in lieu of the switched RC network.
Programmable filters are known that include a switched network of resistors (FIG. 1) and capacitors (FIG. 2) that are switched in and out of the filter circuit depending upon the frequency response desired from the filter, (FIG. 3 and FIG. 4, respectively). A RC network may include a voltage divider circuit that includes a number of resistors (e.g., 2 or more) with intermediate nodes provided between them that are coupled to output terminals via selection switches. Depending on the frequency response desired, a desired selection switch or switches are rendered conductive to couple the desired node to the output through a desired amount of conductance. Similarly, the RC network may include a large number of capacitors each coupled to the output node via respective selection switches. The capacitor selection switches may be rendered conductive selectively to tune the overall capacitance of the RC network to a desired level.
The various switches typically are provided by MOSFET transistors. The MOSFET transistors, however, each introduce some resistance and capacitance to the RC network because they are not perfect devices. Generally, the ON resistance of the MOSFET switch is lower for larger MOSFET transistors. However, as the MOSFET switch is made larger, its device capacitances also increase, (e.g., Cgd, Cgs, Cdb, and Csb). This leads to a dilemma because the higher OFF capacitance affects the high frequency gain of the filter. This can ultimately limit the performance of the filter. The parasitic capacitance of the MOSFET when switching resistors, and the parasitic resistance of the MOSFET when switching capacitors, adversely effects filter performance.
The following discussion will build on aspects of a high-pass filter, as shown in FIG. 5, since this form is most commonly used in equalizers designed to compensate the typically low-pass nature of a communication channel's physical media. It will be immediately apparent to one schooled in the art that the methods and embodiments described herein may be advantageously applied to low-pass, bandpass and other filter and circuit forms.
FIG. 5 shows the canonical implementation of a high-pass filter in single-ended form. FIG. 6 is a well known differential version of this high-pass filter. The solid curves in FIG. 4 and FIG. 3 (curves 330 and 430 respectively), depict the magnitude of the filter's transfer function H(s) as a function of radian frequency, ω. Throughout the specification, when possible standard engineering variables are used. For example, the complex variable “s” is the Laplace parameter and has both real and imaginary parts, i.e. s=σ+jω. The term H(s) denotes the Laplace transform of a circuit's impulse response and is also referred to as the transfer function. The plots of transfer function magnitude, |H(s)|, as a function of radian frequency, ω, shown in FIG. 4 and FIG. 3, are known as Bode plots and describe the input to output behavior of the high-pass circuit for all frequencies.) As a further example, in response to an input voltage, Vin, a circuit characterized by transfer function H(s), will produce a voltage at the output, Vout, equal to Vin*H(s).
As an introduction, the operation of a filter such as that shown in FIG. 6. may be understood as follows. The resistors R1 and the capacitors C conduct current in response to the input voltages and according to their natures, their relation to each other and to other elements of the circuit. These currents flow to the output nodes, sum, and flow in R2, giving rise to the output voltages. Similar to the prior art of FIG. 2, the capacitive portion of current flowing to output node may be increased or decreased by switching in or out more capacitors, respectively. This gives rise to the change in the transfer function, for example, as shown FIG. 4, curves 410 and 420. Similar to FIG. 1, the resistive portion of the current flowing to the output node may be increased or decreased by switching in or out more resistor segments. This causes the change in the filter transfer function show in FIG. 3, curves 310 and 320. This change of the transfer function's magnitude in response to a user supplied input is commonly referred to as tuning the circuit. Such a filter may also commonly be referred to as programmable and as having a programmable transfer function.
Accordingly, the inventors perceived a need in the art for a filter with programmable frequency response that avoids the need for elaborate switched RC networks. In particular, there is a need for a filter that omits transistors from the RC network altogether.
Furthermore, while differential forms are discussed herein the described methods and invention are not limited to differential circuit configurations. The more complex differential forms described in the exemplary embodiments are an extension of the methods and invention that are applicable to single-input or multiple input forms and are within the capability of one of ordinary skill in the art after understanding the following disclosure.